The invention concerns an improved method for the thermal conversion of methane to hydrocarbons of higher molecular weights.
Among the sources of methane are natural gases and refinery gases. The natural gases may or may not be associated with crude oil; their composition varies quite appreciably according to their source, but they generally contain a percentage by volume of methane between 60 and 95%. This methane is always associated with higher alkanes, which can be or even exceed 6 carbon atom hydrocarbons. Various cryogenic methods enable the gases once they have been freed from water and acid components to be separated into several fractions: nitrogen, liquefied natural gases from which the propane and butane fractions are separated, and a fraction consisting essentially of methane associated with a small quantity of ethane. This latter fraction is either reinjected into the wells to maintain the pressure which makes the crude petroleum rise, or conveyed as a combustible gas by gas pipeline, or else burned at the flare stack.
Other sources of methane are refinery gases which are of multiple origin: crude petroleum first distillation gas, hydroreforming gas, gas from various hydrotreatments, thermal cracking gas, catalytic cracking gas; all these gases contain in various proportions methane associated with a number of other gaseous components, such as light hydrocarbons, nitrogen, CO.sub.2, etc.
Thus, for example, an effluent gas from a fluidized bed catalytic cracking unit comprises, after washing, about 30% by volume of methane. This gas is often fractionated by cooling under pressure to obtain two fractions, one comprising hydrogen, nitrogen, methane and a small part of ethylene, and the other fraction containing the major part of the initial ethylene with ethane, propane and propylene. This latter fraction can be advantageously conveyed to a Dimersol type unit, while the first fraction is returned to the refinery fuel gas network in which it is used as a fuel.
The conversion of methane to hydrocarbons of higher molecular weights is therefore of definite interest: thus, in remote natural gas or associated gas deposits, the conversion of methane to acetylene, ethylene and aromatics makes possible, by known methods, obtaining more easily transportable and/or valorisable liquid fractions.
For example, after separation of any solids formed, the aromatic compounds fraction can be separated, and the gaseous fraction then treated first in acetylene oligomerization and/or cyclisation units and then, after a further gas/liquid separation, the residual gas fraction of high ethylene content is treated in Dimersol type units which enable ethylene oligomers to be obtained.
On the refinery sites themselves, the conversion, even partial, of methane to more easily valorisable products also is of great economic interest.
Various methods of conversion of methane have already been suggested; thus U.S. Pat. No. 4,199,533 discloses a method for obtaining ethylene and/or ethane from methane, by reacting this product with chlorine at temperatures above 700.degree. C. This method has the great disadvantage of using very corrosive gases, such as chlorine and hydrochloric acid, at high temperatures.
Among the thermal cracking processes that can convert methane, the so-called "Wulff process" consists in using refractory contact masses; first, the refractory mass is heated by the combustion in air of a fuel which can be the charge itself and then, secondly, the hydrocarbon to be cracked is decomposed by absorbing heat stored in the refractory material during the preceding period; this is therefore a batch process.
Electric arc and plasma processes are used essentially in the preparation of acetylene; their high electrical energy consumption makes their utilization difficult.
A third type of process, sometimes called autothermal, involves burning a part of the charge to supply the heat required for the cracking reaction; this type of process uses a burner in which about one-third of the hydrocarbon is burnt, the remainder being cracked. In view of the high thermal levels obtained, this type of process essentially produces acetylene and coke.
French patent No. 1,211,695 discloses a combined hydrocarbon pyrolysis method involving mixing methane with hot combustion gases containing no excess oxygen and then injecting into the mixture obtained paraffinic hydrocarbons with more than one carbon atom; according to this method a very small part of the methane can be converted to acetylene. There does not exist at present any industrial process using controlled heat transfer through a wall for converting methane to easily valorisable hydrocarbons, such as acetylene, ethylene and/or aromatic compounds.